As this type of conventional rotation/linear motion converting mechanism, a mechanism using a roller screw described in Japanese Laid-Open Patent Publication No. 10-196757, for example, has been known.
This mechanism is provided with a shaft having a thread on the outer circumferential surface, a nut having a thread on the inner circumferential surface, and rollers interposed between the outer circumferential surface of the shaft and the inner circumferential surface of the nut and screwed with each of the above threads. The nut and the rollers are meshed with each other by a gear. When the above nut is rotated, the rollers rotate and revolves around the shaft, that is, perform a sun-and-planet motion, while the shaft screwed with the thread of the rollers makes a linear motion in the axial direction.
A lead in the above rotation/linear motion converting mechanism (stroke quantity of a shaft per rotation of a nut) is determined by the number of threads provided on the shaft, nut and rollers, respectively and reduction gear ratio of the rotation/linear motion converting mechanism. Among them, the reduction gear ratio is determined by a ratio of effective diameters of the respective threads, but the actual effective diameters of the respective threads might be varied due to machining accuracy of the thread or changed due to abrasion on the contact surface between the threads to be screwed together, and a stable constant reduction gear ratio is difficult to be obtained.
Also, in the mechanism described in the above document, since the position of a center axis of the shaft with respect to the center axis of the nut and the position of the revolution axis of the rollers with respect to the center axis of the nut are determined by meshing of the threads, the center axis of the shaft and the revolution axis of the rollers are easily displaced from the center axis of the nut. If the center axis of the shaft or the revolution axis of the rollers is displaced from the center axis of the nut in this way, the position of the contact surface of each thread is changed, which causes change in the actual effective diameter of each thread and a stable constant reduction gear ratio cannot be obtained in this case, either.
If a stable constant reduction gear ratio cannot be obtained in a rotation/linear motion converting mechanism in this way, it becomes difficult to achieve the above lead according to a design value, which deteriorates lead accuracy.